Messages in this thread | | | Date | Wed, 20 Sep 2006 09:20:08 -0400 | From | "Frank Ch. Eigler" <> | Subject | Re: [PATCH] Linux Kernel Markers 0.2 for Linux 2.6.17 |
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Hi -
> > [...] For the static part of the instrumentation, a > > marker that could be hooked up to either type of probing system was > > desirable, which implies some sort of run-time changeability. > > Ok. So if I get what you're saying here, you'd like to be able to > overload a marker?
Sort of. Remember, we discussed markers as *marking* places and things, with the intent that they be decoupled from the actual *action* that is taken when the marker is hit.
> Can you suggest a macro that can do what you'd like. [...]
Compare the kind of marker I showed at OLS and presently supported by systemtap. Its unparametrized version looks like this:
#define STAP_MARK(name) do { \ static void (*__mark_##name##_)(); \ if (unlikely (__mark_##name##_)) \ (void) (__mark_##name##_()); \ } while (0)
A tracing/probing tool would hook up to a particular and specific marker at run time by locating the __mark_NAME static variable (a function pointer) in the data segment, for example using the ordinary symbol table, and swapping into it the address of a compatible back-end handler function. When a particular tracing/probing session ends, the function pointer is reset to null.
Note that this technique:
- operates at run time - is portable - in its parametrized variants, is type-safe - does not require any future technology - does impose some overhead even when a marker is not active
- FChE [unhandled content-type:application/pgp-signature] | |